1. Field of the Invention
The invention relates to a process for preparing 3-dihalomethylpyrazole-4-carboxylic acid derivatives of the formula (I) by reacting α-fluoroamines of the formula (III) in the presence of Lewis acids with acrylic acid derivatives of the formula (II) to give vinamidinium salts of the formula (IV) and the subsequent reaction thereof with hydrazines, and to the vinamidinium salts of the formula (IV) themselves.
2. Description of Related Art
3-Difluoromethylpyrazole-4-carboxylic esters, -carboxamides and -carbonitriles are important synthetic units for preparing active agrochemical ingredients, especially for preparing pyrazolylcarboxanilide fungicides.
WO-A-05 042 468 teaches a process for preparing 2-dihaloacyl-3-aminoacrylic esters by reacting acid halides with dialkylaminoacrylic esters, and the reaction thereof with hydrazine derivatives to give 3-dihalomethylpyrazole-4-carboxylic esters.
WO-A-03 051 820 teaches the preparation of 2-haloacyl-3-aminoacrylic esters by reacting N-substituted 3-aminoacrylic esters with haloalkylcarboxylic anhydrides and the subsequent reaction thereof with hydrazine derivatives to give 3-haloalkylpyrazole-4-carboxylic esters. The reaction to give the 3-haloalkylpyrazole-4-carboxylic esters proceeds unselectively at room temperature and therefore has to be performed at low temperatures (−80° C.).
WO-A-06 005 612 teaches a process for preparing ethyl 4,4-difluoro-3-oxobutyrate by reacting 2,2-difluoro-N-dialkylacetamide with acetic esters in the presence of bases. The ethyl 4,4-difluoro-3-oxobutyrate is subsequently, as described in JACS, 73, 3684 (1951), reacted with trimethyl orthoformate and acetic anhydride to give ethyl (2-ethoxymethylene)-4,4-difluoromethylacetoacetate, which, according to U.S. Pat. No. 5,489,624, can be converted with methylhydrazine to ethyl 3-difluoromethyl-1-methyl-4-pyrazolecarboxylate. The route described firstly includes a multitude of reaction steps, and, secondly, the 2,2-difluoro-N-dialkylacetamide used is not commercially available and can be obtained only in low yields of approx. 70% by fluorinating 2,2-dichloro-N-dialkylacetamide.
The processes described to date in the prior art have the disadvantage that the carbonyl halides, haloalkylcarboxylic anhydrides and haloacrylic esters used are expensive, cause corrosion problems and/or can be purified only with a high level of technical complexity.